1. Field of the Invention
This invention relates to a device for measuring cloud height which operates according to the principle of an optical radar by determining the level at which reflections or echo signals are obtained from water droplets in the atmosphere and using such determinations to indicate the existence of a cloud. The transit time between a light pulse being emitted from the device and the receipt of an echo signal from the atmosphere indicates the level above the device (and thus the height) of the cloud.
Cloud height measuring devices of the type described can also be used where the visibility is impaired but the concentration of water droplets in the atmosphere is not high enough to define cloud conditions. However to classify the visibility in the case of mist it is necessary to know accurately what is the level of attenuation in the light pulses reflected from a particular height so that the vertical visibility in the case of mist can also be calculated.
The transmitter and receiver of a device to which this invention relates are accommodated in a so-called transceiver, which also includes the necessary optical and electronic equipment. The emitted light passes upwards through an output window in a protective housing of the transceiver and the reflected light passes downwardly through a receiver window in the same housing before it is led to a signal detector via a receiver optic. Pollution and/or water droplets on either of the windows of the housing cause the light pulses to be attenuated to varying degrees and make it difficult or impossible to correctly identify the echo signals from a cloud or the determination of the visibility in the case of mist.
The present invention relates to a method for controlling the emitted measurement energy in a cloud height measuring device such that signal attenuation caused by rain and/or dirt on a window of the device is compensated for, and also to a device for carrying out the method.
2. Description of the Prior Art
The determination of cloud height by measuring transit time is performed by measuring the time from the emission of an incident light pulse until reflections exceeding specified amplitude are received back at the receiver. During its transit, the light pulses are scattered and attenuated by obstacles of different kinds, which results in the received measurement energy being considerably lower than the emitted energy. When a transmitter of inherently low power output is used, for example a pulsed GaAs laser, some form of signal filtering is required for detecting clouds within the height range that is of interest in the particular case of aircraft flights.
When the measurement energy in the received reflections becomes low, it may be difficult to distinguish the measuring signal from other signal noise. To be able to perform reasonably reliable measurements in a low signal to noise ratio, it is, for example, possible to employ the method described in U.S. Pat. No. 3,741,655. This method comprises dividing the measurement range into a number of height intervals or levels. Because it is possible to calculate the transit time for a measurement pulse up to any given level and back to the receiver, a measurement of reflected energy can be carried out at the precise moment when a reflection would be expected if a cloud (or some other reflecting object) is present at that given level. Any measuring signal taken at the indicated moment will comprise both a reflection signal and a noise signal. By performing a similar measurement just before the looked-for reflection signal is due, a value is obtained which represents the noise level only. A number of such measurements can be integrated in separate integrators, the output signals of which are finally compared. If a significant difference is observable between receiver output during the time when reflections are expected and during times when they are not, this signifies that a cloud is present within the height interval under consideration. Since the signal-to-noise ratio increases with the square root of the number of integrated measuring pulses, it is possible, despite the limitations of low transmitter power, to detect a cloud at a relatively great height by integrating a sufficiently large number of measuring pulses for each height interval of interest.
The above-discussed method of measuring cloud height is based on a measuring sequence with a fixed time-controlled sensitivity regulation and/or with a fixed level of emission of measurement energy.